With the advent of solar heating units for heating water, it is desirable to provide control systems for controlling the flow of water between a solar collector heater and a hot water storage unit. When the solar collector is receiving sufficient sunlight to heat the water to a temperature above that in a hot water storage, then it is desirable and efficient to move the water in a flow from the solar collector to the hot water storage. There are other times however when it is undesirable, such as at night or when there is little sunshine or when the ambient temperature is cold, to move water from the solar collector to the hot water storage. At the latter times, it is desirable to de-energize the pump pumping water between the solar collector and the hot water storage. Also it is desirable to selectively energize the pump only when there is a given temperature differential between the water leaving the solar collector and the water in the hot water storage. Further it is desirable to turn off the pump when the difference between the temperature of the water leaving the solar collector and that in the hot water storage is less than temperature differential set to energize the pump. This prevents oscillatory energizing of the pump motor with minute changes in relative temperatures of the water in the solar collector and in the hot water storage. Still further it is also desirable to have means for separately detecting the temperature of the water in the solar collector so that regardless of the differential in the temperature between the water in the solar collector and the hot water storage, any time the water in the solar collector falls below a given temperature, such as for example 70.degree., the pump is de-energized.
It is therefore advantageous to have a differential temperature control for selectively energizing and de-energizing a pump motor at the appropriate times, and for the appropriate temperature differentials between water in the solar collector and water in the hot water storage. It is further advantageous to have a efficient circuit for accomplishing the foregoing.